Pericytes grip and support the blood vessel. Graphic courtesy Wendy Brooks, DVM

The hemangiopericytoma is a common tumor in dogs, usually arising on the limbs and appearing as a round nodule under the skin. It derives from a cell called a pericyte that acts to support blood vessel capillaries throughout the body (see below). A hemangiopericytoma develops when a pericyte somewhere in the body develops a chromosomal abnormality (either a missing chromosome or an extra one) and begins to proliferate and invade other local tissues.  The hemangiopericytoma is classified as a type of soft tissue sarcoma and is considered malignant because of its ability to invade and obliterate local tissues; however, it rarely spreads to distant body sites. Hemangiopericytomas can appear bald, pigmented, or even ulcerated. In later stages, the hemangiopericytoma can be quite large and is sometimes mistaken for a benign lipoma (fatty tumor) if tests are not performed.

What is a Pericyte?

Consider that everyone began life as a single cell. This cell divided over and over and became many cells. As time continued, we became groups of different types of cells. Eventually, we became embryos, fetuses, and finally infants with complete organ systems and different tissues all derived from one original cell. During the course of this amazing development, some cells do not specialize fully in specific tissues. They stay "undifferentiated" and can in the future specialize into whatever type of cell our body might require at that time. We have cells throughout our bodies that simply stay put at different stages of embryonic development. Pericytes are such cells. They have arrested their development from embryonic times and are located all over the body, supporting blood vessels and waiting for a signal to develop. If the body has a need, pericytes can develop into new muscle tissue or new blood vessel tissue. A pathology instructor once described the pericyte as perhaps not the Adam and Eve of our bodies but more like Abraham: not the first or even second generation by any means, but ancient.

What is a Hemangiopericytoma?

A hemangiopericytoma is a tumor arising from pericytes associated with blood vessels. Instead of forming a useful tissue, these pericytes form tumors because improper cell division leaves them with chromosome damage.

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Photo of a hemangiopericytoma tumor on a dog's lower leg. (The forceps are included to show size.) Photo courtesy of: Dr. Duane Moore, DVM

Is it Cancer?

The hemangiopericytoma does not spread through the body in the way you normally think of cancer. It does, however, tend to recur at its original site. When it recurs, it tends to grow back more deeply rooted and more difficult to remove than it was originally.

If left alone it will continue to grow inexorably, usually over years, until it invades a part of the body that does not have extra space to allow a tumor; usually, the chest cavity is the closest such area.

What Are the Treatment Options?

Treatment centers around local tumor control, and this usually means surgery. The hemangiopericytoma has microscopic tentacles, making it hard to remove completely. Plus, the fact that it is a blood vessel tumor means that bleeding can be intense and difficult to control.  The goal is to remove as much of the tumor as possible without risk. The first step involves some sort of biopsy, either an attempt at removing the whole tumor or simply a mission to take a small piece and identify the tumor depending on the tumor size and amount of bleeding encountered. Once the growth is confirmed as hemangiopericytoma and the bulk of the tumor has been removed, the focus becomes slowing its growth and preventing recurrence. Here are the options after the initial surgery.

• Radiation therapy
  The hemangiopericytoma is responsive to radiation therapy. This means that the cure rate after radiotherapy is 80-90%, assuming the tumor has been reduced to microscopic size after surgical removal. Only the tumor site receives radiation in this type of therapy, so the patient does not become ill, though the area of radiation exposure will probably lose the ability to grow hair.

• Effective (sometimes called "definitive") radiation therapy, unfortunately, requires 15-18 treatments, and the total cost can easily run $4000-$4500. For radiotherapy to be effective, the tumor must first be surgically removed, leaving no visible remaining tumor. Otherwise, the tumor will be too big for a definitive response. 

  If the goal is not to cure the tumor but to simply slow its recurrence, palliative radiation is an option. This costs substantially less and typically involves a month of weekly treatments. This might not be able to prevent regrowth but could certainly postpone it. Given that the tumor is slow growing to begin with, postponing regrowth might be as good as a definitive response.

• Leave tumor alone for now and attempt another surgical removal when it recurs.
  One option is to wait and see if the tumor regrows (it usually does). When it regrows, it can be re-removed; however, when it re-grows, it is usually more difficult to remove. Every time it is removed, it re-grows more invasively making the next surgery more difficult. If a second surgery removes the visible portions of the tumor successfully, radiation therapy can again be considered as above with similar success rate.
  
  
• Leave tumor alone, let it grow back, do nothing.
  This may be a realistic option for an elderly patient as the hemangiopericytoma typically requires 6-12 months to grow back and longer to become disfiguring or invasive. An older patient may die of old age before the hemangiopericytoma comes back in a meaningful way. For a younger patient, however, this is probably a poor option as the tumor will eventually become, as mentioned, inoperable, disfiguring, and lethal. In this situation, a more definitive plan is better.
  
  
• Allow the tumor to recur, then amputate the limb (assuming the tumor is on a limb)
  This may seem extreme, but it certainly resolves the lethal aspect of the tumor in a curative manner. The hemangiopericytoma does not spread distantly in the body, so amputation of the limb achieves a cure and does so far less expensively than radiotherapy, though amputation is a major surgery and is not without substantial expense and adjustment to the three-legged lifestyle.
  
  
• Metronomic Chemotherapy
  Metronomic chemotherapy involves the use of chemotherapy drugs at very small doses daily or at least much more frequently than the traditional chemotherapy protocols would have them used. For incompletely removed hemangiopericytomas, metronomic chemotherapy aims to hamper the regrowth of the remaining tumor's blood supply. The most common protocols use a combination of cyclophosphamide and piroxicam. This type of treatment is not generally helpful when a grossly visible tumor is present but is best for preventing the re-growth of a tumor after all its visible portions have been removed.

What about Inoperable Tumors?

Some tumors are just too big, are located in a body area not amenable to surgery, or the patient has concurrent problems that make surgery problematic. If surgery is not practical, there are still treatment options to consider:

• Palliative radiation therapy
  As described above, a less aggressive radiation plan can be used to retard the growth of the tumor. This can be very helpful in managing pain, preventing ulceration, and slowing enlargement of the mass. A typical plan might be weekly treatments for a one-month period.
  
  
• Electrochemotherapy
  This newly emerging technique is for managing solid tumors without surgery. Here, electricity is used to temporarily generate pores in the tumor through which chemotherapy can penetrate. This method provides much more efficient chemotherapy penetration than simply letting the tumor take up the chemotherapy agent naturally. The strength of the electricity can also be strong enough to damage the tumor. At this time, electrochemotherapy is only available in some facilities.

Not all veterinarians are comfortable treating a hemangiopericytoma. Discuss with your veterinarian whether a referral to a specialist would be best for you and your pet.